1. Field of the Invention
The present invention relates to a method of adjusting the tension of pile warp yarns according to weaving modes on a pile fabric loom (pile loom). In this specification, a direction from a let-off mechanism toward a take-up mechanism is referred to as the "forward direction" and the opposite direction is referred to as the "backward direction". The term, "fast pick" signifies a pick driven completely from a shed into the cloth fell of a woven fabric on a loom by beating, and the term, "loose pick" signifies a pick driven halfway and not beaten completely into the cloth fell.
2. Description of the Related Art
A method of controlling the tension of pile warp yarns on a shifting fell type (moving cloth type) pile fabric loom and a device for carrying out the method are disclosed in JP-A No. 2622685)(U.S. Pat. No. 4,884,597. FIG. 1 shows a pile warp tension regulating device 1 disclosed in the foregoing cited reference, and FIG. 2 illustrates a process of forming a pile during a weaving operation for weaving a three-pick towel fabric (three-pick pile fabric) by way of example.
Referring to FIG. 1, the pile warp tension regulating device 1 regulates the tension of pile warp yarns 2 so that the pile warp yarns 2 are fed to the cloth fell 4a of a pile fabric (terry cloth) 4 at a desired tension. The pile warp yarns 2 released from a pile warp beam 6 travel via a guide roller 7 and a tension roller 8 to the cloth fell 4a of the pile fabric 4. A tension lever 9 has one end rotatably supporting the tension roller 8, and the other end fixedly connected to the output shaft of a motor 10, i.e., rotative drive source. The torque, the rotating speed and the angular stopping position of the output shaft of the motor 10 are controllable.
The pile fabric loom is of, for example, a shifting fell type which moves the cloth fell of the fabric forward and backward to form piles. The pile fabric 4 moves forward and backward on the pile fabric loom during a weaving operation and the tension lever 9 turns to move the tension roller 8 forward and backward accordingly to permit the pile warp yarns 2 to move forward and backward. Although not shown in FIG. 1, the pile fabric loom is provided with a ground warp beam, a tension control device for controlling the tension of ground warp yarns 3, a take-up control device for controlling a take-up operation for taking up the pile fabric 4, a shedding mechanism, a terry motion mechanism for moving the cloth fell 4a of the pile fabric 4, and such.
While the pile fabric loom is in a weaving operation, a warp beam driving unit 11 drives the pile warp beam 6 to let off pile warp yarns 2. The warp beam driving unit 11 has a speed calculating unit 14 and a current amplifier 15. The speed calculating unit 14 receives a displacement signal expressing a measured displacement of the tension roller 8 measured by a displacement sensor 12 and a desired displacement signal provided by a desired displacement setting device 13, and provides a speed signal expressing the deviation between the displacement signal and the desired displacement signal, the current amplifier 15 amplifies the speed signal to provide a drive signal for driving a let-off motor 16 for operation to let off the pile warp yarns 2.
The motor 10 for driving the tension lever 9 for turning is controlled by a pile warp tension controller 20. The pile warp tension controller 20 includes a driving unit 22, a tension control signal generating unit 17 which generates a tension signal for making the motor 10 generate a predetermined torque, a speed control signal generating unit 18 which generates a speed signal for driving the motor for operation at a predetermined operating speed, and an stopping angular position control signal generating unit 19 which generates an stopping angular position control signal expressing an angular position at which the output shaft of the motor 10 is to be stopped. The tension signal, the speed signal and the angular stopping position control signal are given selectively to the driving unit 22 by selectively closing switches 21.
The switches 21 are closed selectively according to the mode of weaving operation to send the speed signal to the driving unit 22 to weave a fast pick in the loop forming cycle when the pile fabric loom operates in a fast pick weaving mode, to send the tension signal to the driving unit 22 to weave a loose pick in the loop forming cycle when the pile fabric loom operates in a loose pick weaving mode and to send the angular stopping position control signal to the driving unit 22 when changing the mode of weaving operation from the fast pick weaving mode to the loose pick weaving mode and vice versa. The driving unit 22 executes a speed control operation, a tension (torque) control operation or an stopping angular position control operation according to the signal given thereto.
The pile warp yams 2 are controlled at a relatively low first tension for the loose pick weaving mode for forming piles (loops), and at a relatively high second tension for the fast pick weaving mode for weaving the ground fabric. The ground fabric includes all the parts of the pile fabric excluding piles, such as borders and hems. FIG. 2 is a diagrammatic view comparatively illustrating the operating speed of the pile fabric loom, operation periods of the loop fabric loom, control modes of the pile warp tension control unit 20, and the variation of the tension of the pile warp yarns for weaving a three-pick towel fabric (three-pick pile fabric). One complete pattern of the weave of the three-pick towel fabric (for forming one loop) is completed by inserting three picks, i.e., by three rotations of the crankshaft of the pile fabric loom. Since the pile fabric loom is of a shifting fell type, picks (weft yarn which is inserted into shed) are beaten at a fixed beating position and the pile fabric 4 is moved once forward and backward every three picks, ie., every three rotations of the crankshaft of the loop fabric loom to form piles of a desired length between the beating position and the cloth fell 4a of the pile fabric 4.
Referring to FIG. 2, piles are formed in a state (1), the cloth fell 4a is shifted forward to the front position to insert loose picks in states (2), (3) and (4), and the cloth fell 4a is shifted backward to the back position to weave a fast pick in states (5) and (6). In operation periods R1, R2 and R3, the crankshaft of the pile fabric loom is in the first rotation cycle, in the second rotation cycle and a first half of the third rotation cycle and in the second half of the third rotation cycle, respectively.
In the state (1), i.e., a state immediately after the completion of pile formation, the pile fabric loom is in the rotation period R1, the cloth fell 4a is moved forward by a predetermined distance by a terry motion mechanism, not shown, the pile warp tension control unit 20 executes a speed control operation V to supply a current corresponding to a speed signal to the motor 10, and the motor 10 turns the tension lever 9 forward in the state (2) to displace the tension roller 8 forward by a distance corresponding to the distance by which the cloth fell 4a is moved forward Thus the tension lever 9 is turned prior to the forward movement of the cloth fell 4a to slacken the pile warp yarns 2 temporarily. Consequently, the tension of the pile warp yarns is reduced to a level which will not cause mislooping, i.e., failure in forming piles.
Then, the pile warp tension control unit 20 starts an angular stopping position control operation B, and gives an angular stopping position control signal to the motor 10 to stop the motor 10 so that the tension lever 9 is stopped at a predetermined position. On the other hand, the terry motion mechanism, not shown, moves the pile fabric 4 forward continuously to change the tension of the pile warp yarns 2 from a low tension to a set tension. Since the excessive forward movement of the tension roller 8 is inhibited by the angular stopping position control operation B, the forward turning of the tension lever 9 can be stopped and the tension lever 9 is kept stably in a stopped state.
Meanwhile, head frames 23 are moved for shedding motion to form a shed by the pile warp yarns 2 and the ground warp yarns 3. A picking mechanism, not shown, picks a weft yarn 5 in a first pick 5 indicated at 1 into the shed. The first pick 5 is beaten up by a reed 24 at the back beating position. In this state, the loom operates to produce a loose loose pick.
In the rotation period R2 for the states (3), (4) and (5), a second and a third pick 5 indicated at 2 and 3 in FIG. 2 are picked, and the terry motion mechanism moves the pile fabric backward to form piles. Meanwhile the pile warp tension control unit 20 executes a tension control operation T and a current corresponding to a desired tension signal is supplied to the motor 10. Consequently, the tension lever 9 is turned forward and backward according to the shedding motion so that the torque of the motor 10 and the sum of the tensions of the pile warp yarns 2 balance each other. Thus, the tension of the pile warp yarns 2 are maintained always at a set tension.
In the rotation period R3, the pile warp tension control unit 20 executes an angular stopping position control operation B to stop the tension roller 8 by braking the motor 10 in the state (6). That is, two picks 5 have been inserted in the shed in the states (2) and (3), then the first pick state (5), namely, the cloth fell 4a is moved backward to the beating position, a third pick 5 indicated at 3 is inserted, and thereafter the tension roller 8 stops as shown in the state (6) so that the third pick 5 is beaten by the reed 24. Consequently, the pile warp yarns 2 held by the two picks 5 form piles of a predetermined length corresponding to a reed clearance, ie., the distance of forward movement of the cloth fell 4a from the beating position in the state (6).
In this one pile forming cycle of the pile fabric weaving (pile weaving) operation, the three picks 5 are inserted and each pile warp yarn 2 forms one pile while the crankshaft of the pile fabric loom makes three full turns. When the fast pick is woven in the pile forming cycle, the tension of the pile warp yarns 2 are kept at a desired relatively low tension by the tension control operation T. When a ground fabric weaving (plain weaving) cycle (for example, a border weaving or hem weaving) is started after the pile forming cycle has been repeated a predetermined number of times, the cloth fell 4a is not shifted and fast picks are woven consecutively, the pile warp tension control unit 20 executes the tension control operation T subsequent to the angular stopping position control operation B to keep the pile warp yarns 2 at a desired tension suitable for weaving the ground fabric.
A general known technique disclosed in JP-U No. 2-74386 sets desired pile warp tension amounts respectively for border weave sections and pile weave sections, and executes a tension control operation to keep the pile warp yarns at the desired pile warp tension to weave pile weave sections. More concretely, the desired pile warp tension for weaving pile weave sections is relatively low and the desired pile warp tension for weaving border weave sections is relatively high. Since the pick density of border weave sections is high, the tension of the pile warp yarns is increased when weaving border weave sections to ensure successful picking, and the same is reduced when weaving pile weave sections.
Problems in the Prior Art
When changing the weaving operation of the pile fabric loom from a pile weave section weaving mode to a border weave section weaving mode, the prior art increases the tension of the pile warp yarns instantaneously while the pile warp yarns are not effectively gripped by the ground warp yarns and the picks and, consequently, a high tension for the border weave (section weaving mode is exerted on the loose piles formed by reducing the tension of the pile warp yarns and thereby the piles formed in the pile, weave section are stretched and mislooping results.
Since the yarn density of the ground weave greater than that of the pile weave sections, the cloth fell moves backward after beating during the weaving operation for weaving the ground weave sections. The reed clearance is reduced by such backward movement of the cloth fell after beating even if the tension of the pile warp yarns is reduced when changing the weaving operation from the ground weave section weaving mode to the pile fabric weaving mode. Consequently, it is possible that the quality of the pile fabric is deteriorated by short piles of a length shorter than the desired pile length formed immediately after the change of the operating mode of the pile fabric loom from the ground weave section weaving mode to the pile weave section weaving mode and by failure in forming piles.